1. Field of the Invention
The present invention is directed generally to the field of drive circuits and more specifically to drive circuits for thin film electroluminescent (TFEL) edge emitter devices.
2. Description of the Prior Art
It is well known that electroluminescent devices generally, and, particularly, thin film electroluminescent edge emitting devices, may be utilized to provide an electronically controlled, high resolution light source. TFEL elements emit light when a changing electric field is present across the element. Examples of the structures of TFEL elements are found in U.S. Pat. No. 4,535,341, U.S. Pat. No. 4,734,723, and U.S. Pat. No. 4,899,184.
TFEL edge emitter devices are typically configured in arrays utilizing multiplexed common drive circuitry to control many TFEL devices from a single source. A common driver circuit is typically used to generate the peak-to peak voltage necessary to illuminate the TFEL edge emitter device, and a demultiplexing circuit that directs the signal to the individual devices as desired.
TFEL devices require large operating voltages on the order of five hundred volts peak-to-peak. Not only is a high voltage required for operating the device, but the voltage must be precisely regulated to obtain consistent and reproducible light emission from the TFEL device. Traditionally, the common driver circuits use current steering transistors to charge the capacitive TFEL load. The voltage stored in the TFEL load is then discharged by dissipating the power through resistive circuitry.
More recently, a more efficient common drive circuit has been developed which comprises a bidirectional switch for selectively connecting an individual pixel terminal to a source of voltage through an inductance thereby enabling current to flow into the pixel to charge the pixel to an operating voltage and enable current to flow out of the pixel back to the source of voltage when the pulse is terminated. A transformer has a primary winding connected between the first terminal and the bidirectional switch for enabling the pixel to charge to an operating voltage, and a secondary control winding connected in series with a diode across the source voltage and ground for limiting the value of that operating voltage. The bidirectional switch is operated so that a substantial portion of the energy used to charge the pixel is returned to the source of voltage at the end of the pulse duration. The reader desiring more information concerning the construction and operation of the aforementioned drive circuit is directed to U.S. Pat. application Ser. No. 412,241 filed Sep. 25, 1989, and assigned to the same assignee as the present invention, which is hereby incorporated by reference.
Experimental evaluation of the most recent power saver circuit points out two potential problem areas. First, the voltage of the pulse tends to droop during the duration of the pulse. The droop in pulse amplitude following the leading edge of the pulse may produce non-uniform pixel excitation in the presence of significant common bus resistance. This is caused by discharge of the load capacitance through the currently utilized commutator level shifter and by the resistance component of the pixel impedance. Second, the duration of the trailing edge of the pulse when compared to the duration of the leading leading edge of the pulse may be too long in order to achieve optimum performance. There is a need for an improved power saving circuit that will rectify these problems existing in power saver circuits.